The present invention relates to a primary air and fuel supply system for a combustion chamber, in particular a combustion chamber for a turbojet engine.
Conventional turbojet combustion chambers comprise a primary zone, having a high fuel/air ratio, and a dilution zone, located downstream of the primary zone in which the fuel/air mixute is diluted by mixing it with additional air. The primary air flow passes into the primary zone through external and internal swirl vanes located around the fuel injector so as to create a cone of atomized fuel leaving the injector. The remaining primary air enters the combustion chambers through orifices or openings in the upstream end of the chamber, and through openings in the inner and outer walls of the combustion chamber. The amount of air passing into the primary zone as a percentage of the total air flow from a compressor in most instances is a trade off between the optimum performance level requested of the combustion chamber at full power and the optimum performance requested at idle speed. The performance characteristics at full power require minimum smoke emission and an even temperature distribution throughout the chamber, while the performance requirements at idle are somewhat different so as to promote an efficient, stable idle characteristic.
In view of the higher performance levels required of the combustion chambers in modern turbojet engines, the trade offs between the idle requirements and the full power requirements have become increasingly difficult to achieve. One attempt at resolving this problem has been to design the combustion chamber with two modules; one module designed for full power applications; the other being designed for idling conditions. However, these chambers, in addition to being bulky and costly since they require double the injection points, have also encountered problems in achieving optimum performance in the intermediate power levels between idle and full power.
Another solution which has been incorporated into both the single and the two-module combustion chambers, comprises movable shutters which act as diaphragms to continuously match the air flow distribution of the combustion chamber air intakes to the desired power head such that the operation of the chamber can be continuously optimized. Typical examples of such movable control diaphragms are disclosed in French Pats. Nos. 2,491,139 and 2,491,140. These devices have the disadvantages of poorly guiding the air at the intake of the swirl vanes and also generate large wakes within the combustion chamber.
Aerodynamic, bowl-type injectors have been developed, such as described in U.S. Pat. No. 4,162,611 to Caruel et al. The injector is mounted in the upstream end of the combustion chamber and is surrounded by a bowl-shaped member having a frusto-conical portion flaring outwardly in the downstream direction, and having an end wall perforated by several small-holes through which highly pressurized air enters the atomized fuel cone. Because of the turbulence created by the bowl and the resultant thorough mixing of the atomized fuel, a mini-primary zone is created during idle which promotes the optimum operating characteristics of the combustion chamber.
To improve the intermediate bowl-shaped aerodynamic injectors, the outer swirl vanes, as well as the air intake for the bowl orifices have been equipped with a control diaphragm to modulate the air flow to match the air-fuel mixture proportions at the bowl outlet for all operational modes of the combustion chamber and to match this fuel richness to all intermediary states between idle and full power. Such a design is shown in U.S. patent application Ser. No. 792,685 entitled "Variable Flow Air-Fuel Device for a Turbojet Engine" filed on Oct. 29, 1985.